Alphaviruses, which can cause diseases ranging from infectious arthritis to lethal encephalitis, are a significant cause of human disease. Though the molecular pathogenesis of alphavirus infections has been extensively studied, relatively little is known about how these viruses interact with and evade the host immune response. In studying the adult mouse neurovirulent AR86 strain of Sindbis virus; our laboratory has identified several determinants of adult mouse neurovirulence within the viral nonstructural protein coding region. One of these determinants, at nsP1 538, interacts with the host type I IFN system, since viruses with an attenuating mutation at this position induce significantly more type I IFN than the wild type virus in vivo and in vitro. Further analysis demonstrated that the AR86 nonstructural proteins are able to directly interfere with RIG-I mediated type I IFN induction by specifically interfering with the cytoplasmic RNA sensor, RIG-I. The demonstration of specific antagonism of the RIG-I dependent IFN induction pathway represents the first evidence for specific antagonism of the type I interferon system by alphaviruses. Therefore, studies are proposed to 1) characterize the mechanism(s) by which the viral nonstructural proteins antagonize RIG- I function, 2) determine whether other steps in the type I IFN induction pathway are also targeted by the viral nonstructural proteins, 3) determine which nonstructural proteins or polyprotein precursors mediate the RIG-I antagonism, and 4) determine whether interactions between the virulence determinant at nsP1 538 and the RIG-I/Mda5/MAVS interferon induction pathway contribute to the regulation of type I IFN induction by AR86, with subsequent effects on viral virulence. This work has the potential to significantly advance our understanding of alphavirus pathogenesis by dissecting the mechanisms by which these important human pathogens interact with the host innate immune system. Alphaviruses represent a significant emerging threat to human health, and an increased understanding of how these viruses interact with and subvert the host innate immune response is likely to assist in the development of improved vaccines/therapeutics against these pathogens. Furthermore, since alphavirus-based vectors are in development as vaccine delivery platforms, the increased understanding of how these viruses interact with the innate immune system is likely to result in the generation of safer and more immunogenic vectors.